Naturally occurring clay minerals are regularly used in many applications. For example, catalyst, paper, plastic, health care, petroleum exploration and adsorbent applications are known. Of particular interest are clays known as swelling clays. Typical of these are the smectite clays. The term "swelling" refers to the expansion of the clays in the C-dimension of its natural crystalline state, when exposed to water. A schematic representation of a swelling clay, sodium montmorillonite, is illustrated in FIG. 1.
In the fully expanded state, the surface area of swelling clays is theoretically calculated to be of the order of 750 m.sup.2 /g. In practice this surface area is not available for reaction when clays are heated above 150.degree. C. The reason for non-accessibility is the diffusion of interlayer solvent out of the clay layers at higher temperatures, causing strong layer to layer Van der Waal contacts. To circumvent this, a number of modifications have been proposed. Among the most successful is the intercalation of metal hydroxy polymer cations followed by heating above 200.degree. C. In the literature this process is referred to as pillaring or cross-linking. Among the most widely explored, inorganic polymers were synthesized by hydrolysis of water soluble salts of aluminum, iron, chromium, bismuth, magnesium, zirconium, and nickel at an appropriate pH. In addition, references are known where pillaring species are derived from molybdenum, niobium, silicon and other metal ions.
Commonly, pillared clays are prepared by ion exchange of cationically charged metal hydroxy polymers with sodium ions of clays. The typical exchange reactions are conducted at temperatures around 70.degree. C. for three hours or at room temperature over a longer period of time. In the less common method, metal hydroxy polymers are synthesized by an in situ method in the interlayers. In this case, known amounts of metal salts and an inorganic base, sodium hydroxide, are stirred with clays at room temperature for a time longer than 10 hours. Typically the ratios of hydroxyl groups to metal are in the range of 1.5 to 3.0. It has been suggested that in this range of OH/metal ratios the polymers formed are of the biggest possible size. For pillaring, bigger size cations are preferred for two reasons. First, bigger cations interact more strongly with clay layers and thus would bond preferentially. Secondly, bigger cations provide higher C-dimension expansion.
In almost all of the work described in the literature, pillaring or cross-linking of clays is carried out by intercalation of only a mono-metallic metal hydroxy polymer. This invention demonstrates pillaring of clays from a different perspective: to create multi-metallic intercalants. In the subject process, two distinctly different, chemically as well as physically, metal hydroxy polymers are intercalated in the same interlayer voids, see FIG. 2A. In addition, pillaring is also carried out using a discretely synthesized multi-metallic hydroxy polymer prior to cation exchange reactions, see FIG. 2B. Such products are designated as mixed pillared clays (MPC). A schematic representation of both types of MPCs is provided in FIGS. 2A and 2B respectively.
Objects of the present invention are to provide techniques of producing novel pillared clays instrumental in enabling specific catalytic reactions to be carried out by virtue of the highly structured interlayers of the clays; to provide unique ways of producing high surface area clays where one pillar acts simply as an inert prop and the other pillar can be used to carry out catalytic reactions; and to enable generating metallic clusters of size that cannot be produced by other techniques. The small metallic clusters are extremely reactive and very important in catalyst applications. The use of multi-metallic discrete metal complexes would allow almost mono-atomic separation of active metals sites in an inert matrix.
Further objects are to develop preparation methods for synthesizing mixed pillared clays containing separate, discrete iron pillars and separate, discrete aluminum pillars and to carry out reduction of iron in these mixed pillared clays with emphasis on the synthesis of small iron crystallites; and thereby obtain reduced mixed pillared clays of good stability at elevated temperatures.
In U.S. Pat. 4,176,090 to David E. W. Vaughan et al, Example 13 describes an Al-Mg polymer for interlayering smectite. However, the method of making this material is significantly different, i.e., it is solid state polymerization. There is no disclosure either of preparing a discrete, multimetallic hydroxy polymer prior to cation exchange reaction with a swelling clay or of preparing two different metal hydroxy polymers and mixing them in desired proportions with the swelling clay.
In U.S. Pat. 4,271,043 to David E. W. Vaughan et al at column 5, lines 19-25, the term "copolymerizing" is used in a different sense. The actual purpose is to stabilize the colloidal system by addition of small amounts of sodium silicate and other compounds. As to what is absent from the disclosure, the above comments apply. A similar description is found in U.S. Pat. 4,248,739 at column 2, lines 30-33 and 55-62. Once more, this is not true copolymerization but the addition of separate cations to already polymerized species, see Example 12. This is much different from actual copolymerization.
Sancier and Inami in J. Catalysis, Vol. 11, 135 (1968) reported reduction of supported iron oxides to metallic iron in the presence of either Pd or Pt via hydrogen spillover. The catalyst samples containing Pt (0.5%) and Fe.sub.2 O.sub.3 (0.05% Fe) supported on either Al.sub.2 O.sub.3 or SiO.sub.2 were prepared by impregnation. Upon reduction of these samples in hydrogen at 770K for 16 hours, their ESR results indicated the formation of the metallic iron domains (i.e., g value of 2.10). However, in the absence of platinum the formation of the metallic iron phase was not observed. Based on these observations, Sancier and Inami concluded that the mechanism of the iron oxide reduction occurred by a sequence of events including: (i) hydrogen chemisorption on platinum, (ii) hydrogen transfer to the support and to iron oxide sites and (iii) subsequent coalescing of iron to form the ferromagnetic domains.
Of qeneral interest are U.S. Pats. 4,436,832 and 4,465,892, also U.S. Pat. 4,238,364.
Pillared clays may be utilized as adsorbents, catalysts, catalytic supports and for other purposes as discussed in the literature.